It is known to perform inspection of a printed circuit boards by optical scanning of the printed circuit board. In view of the high resolution required for reliable inspection, typically an image of one or a few lines along the circuit board at a time is captured with a line sensor. The printed circuit board and the line sensor are moved relative to one another in a scanning direction transverse to the linear direction of the sensor. Thus an image of the printed circuit board is progressively scanned.
One important aspect of the design of such an inspection apparatus is the illumination system. High demands are placed on this illumination system. It must provide light of high intensity on the line on the printed circuit board that is being scanned. At the same time, this light should be such that oblique surface parts of the printed circuit board, for example near the edges of conductor track will not lead to inspection errors.
U.S. Pat. Nos. 4,877,326 and 5,058,982 disclose illumination systems for PCB inspection that use a central linear light source configured to illuminate a line on a substrate from the direction of the sensor. In addition auxiliary linear light sources are provided on either side of the central linear light source. Curved mirrors are provided to image the auxiliary light sources onto the substrate, so that light from each auxiliary light source arrives from a range of directions along the line on the substrate.
From the different art of document scanning, it is known from U.S. Pat. No. 5,825,945 to illuminate a document by means of a Lambertian cylinder, that is, a cylinder with diffusively (not specularly) reflective inner walls. A light source is provided in the cylinder. The cylinder has a slit that passes light from the interior of the cylinder to a line on the document where the document is scanned.